1. Field of Invention
This invention relates to pressure-activated material whose electrical characteristics are modified in response to a pressure applied to the material.
2. Description of the Related Art
Pressure-activated electrically conductive materials of the prior art include various electrically conductive fillers embedded within an elastomeric polymeric matrix. Such materials, however, are known only to provide for decreasing electrical resistance of the material as a function of increased pressure applied thereto. Many of these devices are designed for either a single use, for operations having a relatively short duration, and/or low current operation.
In accordance with one aspect of the present invention, there is provided a pressure activated electrically conductive polymeric matrix material that is doped with particulate filler material. Electrical conductivity is pressure activated with a change in electrical resistance: specifically, with no pressure applied, the material is at a high resistance and with pressure the resistance is materially lower. Conductive fillers may be spherical or powder substrate such as glass, graphite, etc., having plated thereon a metal coating which is electrically conductive and which is more thermally conductive than the substrate. The conductive filler also may comprise a solid metal substrate where the metal of the substrate is less thermally conductive than the metal coating thereon, a gas-filled hollow metal sphere, or a hollow glass or like sphere which is coated with an outer layer of metal. The polymeric matrix materials may include elastic polyurethane, silicone, and many other synthetic or natural rubbers.
As desired for a given application, the material of the present invention exhibits a unique on-off switching characteristic, in that, at a pressure smaller than actuation pressure, the amount of current the material can switch is zero; at pressure greater or equal to actuation pressure, the material switches the full current. This translates into a very sharp decrease in the electrical resistance of the material with little or no detectable increase in the overall temperature of the material.
In another application, the material exhibits a latching function, in that, when the material is under pressure, current drops immediately to zero or a few milliamperes when the continuous current flowing through the material exceeds its maximum continuous current flow value with no noticeable change in the overall temperature of the material. The differences in electrical and thermal conductivity of the metal coating and the substrate and the small contact areas between conductive spheres or powder particles are believed to be the physical bases of the on-off switching and latching characteristics.